tell tales means to talk about things that should remain secret; a false or unkind account. For example: Children shouldn't tell tales. (= Children shouldn't tell lies about each other.) or tell-tale (adj.) means that makes a fact known, especially an unpleasant fact. For example: The murderer was given away by a few tell-tale bloodstains on his car seat. or telltale (n.) means a person who informs about other people's secrets, wrong actions, etc.

Something is "tell-tale" if it is a sign or evidence of something, sometimes a secret, but not necessarily. It does not refer to a person, but some evidence. "She knew that he had been smoking from the tell-tale odor on his clothes." "His mother knew he'd eaten the cookies because of the tell-tale crumbs on his face."